Fix usage of Thread.handle_interrupt in MonitorMixin#mon_synchronize

* See https://bugs.ruby-lang.org/issues/17669
  and https://github.com/ruby/monitor/issues/2
  and jruby/jruby#6526

Author: eregon

library/monitor/synchronize_spec.rb

@@ -0,0 +1,30 @@
+require_relative '../../spec_helper'
+require 'monitor'
+
+describe "Monitor#synchronize" do
+  it "unlocks after return, even if it was interrupted by Thread#raise" do
+    exc_class = Class.new(RuntimeError)
+
+    monitor = Monitor.new
+    10.times do
+      locked = false
+
+      thread = Thread.new do
+        begin
+          monitor.synchronize do
+            locked = true
+            # Do not wait here, we are trying to interrupt the ensure part of #synchronize
+          end
+          sleep # wait for exception if it did not happen yet
+        rescue exc_class
+          monitor.should_not.mon_locked?
+          :ok
+        end
+      end
+
+      Thread.pass until locked
+      thread.raise exc_class, "interrupt"
+      thread.value.should == :ok
+    end
+  end
+end